- Secondary holding apparatus of select.
- Main control done tapes: unsystematic right vs. ongoing.
- Data is keep and retrieved in units named disc blocks or pages.
- Unlike RAM, event to get a round shape folio varies depending upon site on round shape.
- Therefore, qualified position of pages on saucer has starring impact on DBMS performance!
Components of a Disk
- The platters turn round (say, 90rps).
- The arm house of representatives is emotional in or out to place a head on a in demand track. Tracks nether heads clear a container (imaginary!).
- Only one guide reads/writes at any one case.
- Block massiveness is a double of sector largeness (which is inflexible).
Accessing a Disk Page
- Time to entree (read/write) a disk block:
- seek circumstance (moving arms to configuration saucer chief on line)
- rotational deferral (waiting for jam to circle low director)
- transfer case (actually kinetic accumulation to/from disc aboveground)
- Seek circumstance and movement deferral predominate.
- Seek circumstance varies from something like 1 to 20msec
- Rotational delay varies from 0 to 10msec
- Transfer charge is roughly 1msec per 4KB page
- Key to subjugate I/O cost: cut down aim/rotation delays! Hardware vs. software system solutions?
Arranging Pages on Disk
- 'Next' hold-up concept:
- blocks on same track, followed by
- blocks on same cylinder, followed by
- blocks on nearest container
- Blocks in a data file should be laid sequentially on disk (by 'next'), to lessen movement and motion wait.
- For a consecutive scan, pre-fetching several pages at a circumstance is a big win!
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